We have measured stable isotopic compositions of Miocene pelagic fine-fraction (< 63 μm) carbonates from oligotrophic deep-sea sites in the Pacific and Atlantic oceans and compared them with those of coexisting foraminifers to test their utility as near sea-surface indicators. Fine-fraction carbonates (primarily polyspecific nannofossils) and surface-dwelling planktic foraminiferal (Globigerinoides) stable isotopes both have been considered to reflect surface-water hydrographic conditions. However, our results indicate that fine-fraction stable isotopes are offset from and do not correlate well with those of Globigerinoides. In contrast, stable isotopic records of the deep-dwelling planktic foraminifer Globoquadrina are in good correspondence with the fine-fraction records in terms of long-term (ca. > 1 m.y.) trends and temporal variability. On the basis of a time-series hydrography and flux study site in the oligotrophic subtropical North Atlantic, we interpret the isotopic discrepancies between fine-fraction and Globigerinoides as resulting primarily from season of calcification, as well as possible vital effects. We suggest that fine-fraction stable isotope values from oligotrophic waters reflect late winter-early spring relatively cool, nutrient-rich shallow mixed-layer conditions during the time of deep mixing (i.e., spring bloom), whereas Globigerinoides stable isotope values record conditions that prevailed in the stratified surface waters in the warmer late spring-fall. This implies that paired analyses of fine-fraction anal surface-dwelling planktic foraminiferal-δ18O could be applied to reconstruct paleoseasonality of the open oceans. However, because the fine-fraction δ13C values are not representative of the annual mean surface-water δ13C, we recommend use of near surface-dwelling planktic foraminiferal δ13C as a proxy for δ13C of stratified surface waters that are more or less in equilibrium with the atmosphere with respect to pCO2.
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